Reading a phase change memory

ABSTRACT

A phase change memory cell may be read by driving a current through the cell higher than its threshold current. A voltage derived from the selected column may be utilized to read a selected bit of a phase change memory. The read window or margin may be improved in some embodiments. A refresh cycle may be included at periodic intervals.

BACKGROUND

This invention relates generally to semiconductor memories.

Phase change memory devices use phase change materials, i.e., materialsthat may be electrically switched between a generally amorphous and agenerally crystalline state, as an electronic memory. One type of memoryelement utilizes a phase change material that may be, in oneapplication, electrically switched between generally amorphous andgenerally crystalline local orders or between different detectablestates of local order across the entire spectrum between completelyamorphous and completely crystalline states.

Typical materials suitable for such an application include variouschalcogenide elements. The state of the phase change materials is alsonon-volatile. When the memory is set in either a crystalline,semi-crystalline, amorphous, or semi-amorphous state representing aresistance value, that state is retained until reprogrammed, even ifpower is removed. This is because the programmed resistance represents aphase or physical state of the material (e.g., crystalline oramorphous).

Conventionally, phase change memories are read without triggering thememory element. Triggering occurs when the threshold current (atthreshold voltage) of the memory element is exceeded. As a result, theread current may be limited to avoid triggering. But limiting the readcurrent reduces performance.

Thus, it would be desirable to improve the performance of phase changememories.

The present application is subject to a joint research agreement betweenIntel Corporation and Ovonyx, Inc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram for one embodiment of the present invention;

FIG. 2A is a hypothetical, representative current-voltage curve for athreshold device.

FIG. 2B is a hypothetical, representative current-voltage curve for amemory element.

FIG. 3 is a circuit diagram for the voltage reference generator shown inFIG. 1 in accordance with one embodiment of the present invention;

FIG. 4A is a plot of voltage versus time for a selected column inaccordance with one embodiment of the present invention;

FIG. 4B is the column current for the selected column versus time inaccordance with one embodiment of the present invention;

FIG. 4C is a plot of the sample and hold strobe voltage versus time inaccordance with one embodiment of the present invention;

FIG. 4D is a depiction of the R strobe that latches the cell state orread strobe voltage versus time in accordance with one embodiment of thepresent invention;

FIG. 5 includes read timing diagrams; and

FIG. 6 is a system depiction according to one embodiment of the presentinvention.

DETAILED DESCRIPTION

Referring to FIG. 1, a memory 10 may include an array of memory cells 12arranged in rows 16 and columns 14 in accordance with one embodiment ofthe present invention. While a relatively small array is illustrated,the present invention is in no way limited to any particular size of anarray. While the terms “rows” and “columns” are used herein, they aremerely meant to be illustrative and are not limiting with respect to thetype and style of the sensed array.

The cell 12 may be any memory cell including a phase change memory cell.Examples of phase change memory cells include those using a chalcogenidememory element 12 b and threshold device 12 a. One select or thresholddevice is an ovonic threshold switch that can be made of a chalcogenidealloy that does not exhibit an amorphous to crystalline phase change andwhich undergoes rapid, electric field initiated change in conductivitythat persists so long as a holding current is present.

In the case illustrated, a cell 12 includes an access, select, orthreshold device 12 a, as well as a sensing device 12 b which actuallystores a bit of data. A set of transistors 46 may be located on thecolumns 14 in order to enable write quenching and deselecting, andprecharging to a voltage, which may be different than the deselectvoltage V_(DES) by multiplexing to a different voltage.

A column decoder 18 receives address signals to select the desiredcolumn 14 using transistors 20 associated with each column. A resetwrite current source 22, a set write current source 24, and a readcurrent source 26 are coupled at a common node at their outputs tosupply current to the selected column 14, such as the column 14 b asdetermined by which column transistor 20 is on. Of course, the currentsources in actual practice would be coupled to selected columns asneeded in response to commands from an external memory user such as aprocessor (not shown), at the address provided by the external user.

A row decoder 60 receives address signals to select the desired row. Theoutput of the decoder 60 passes through an inverter made up oftransistors 62 and 64 to the selected row 16.

Transistors 28, 38 and 39 are on/off switches that provide the desiredcurrent generated by the current sources 22, 24 or 26, to the selectedcolumn 14. A NOR gate enable circuit 36 with an input from Enable 34provides a control circuit for the transistor 39 and thus for the readcurrent source 26 and also may provide an enable signal for Din gates 25and 26 to enable one of the two gates for writing to the user selectedstate. The gates 25 and 26 are gated on by an enable circuit 36 andreceive a Din 30 signal as another input to select which write currentsource is activated. An input/output (I/O) control 32 is coupled to theDin circuit 30. Enable 34 is controlled by the write input to the chip.Write input or read fetch 47 requests a read cycle from the on-chiptiming block 49, which starts the on-chip timing and sequentiallyenables the comparator 42 and latches the data for the I/O 32 for aread, or enables Din gate 25 and 26 (and disable gate 36) if a write andproduces a current pulse of appropriate amplitude to write a reset andless amplitude to write a set bit. Alternatively, the current amplitudeof both reset and set bits may be adequate for reset, with the bitwritten to reset if the trailing edge is fast (e.g. less than 10 nsec)or slow (e.g. greater than 100 nsec.) if the set state is desired.

For reading, a comparator 42 is enabled by the on-chip timing 49 (inresponse to a read fetch request 47) and receives one input from aselected column (e.g. column 14 c) being read. The comparator 42 andreference voltage generator 40 may be provided on each column 14 in oneembodiment, but as shown, the generator 40 may be shared across an arrayor block of row or column lines to reduce comparator 42 related layoutarea.

The reference voltage generator 40 receives a voltage VR from the column14 and outputs a voltage VREF to the comparator 42. The referencegenerator 40 is strobed by a sample and hold (SH) strobe from theon-chip timing block 49. The output from the comparator 42 to a dataoutput latch 44 provides an output enable (OE) signal as an option whichat least indicates when the output can be driven (ready/busy), thoughusually OE is furnished by a separate external processor to enable theoutput driver. The output signal from the latch 44 is controlled by aread (R) strobe from the on-chip timing block 49.

The set state corresponds to a lower resistance value and the resetstate corresponds to a higher resistance value in the memory element 12b. Thus, the reference voltage level is between the reset and setlevels, at the instance of time when read data is latched.

Referring to FIG. 3, the generator 40 receives a reference signalvoltage VR from the selected column 14. That voltage is buffered by afirst amplifier 50 and a second amplifier 54. The amplifier 54 receivesits voltage from voltage divider 52. While a resistor divider is shown,other voltage dividers may be used including a capacitor divider forreduced power and better layout efficiency. The voltage divider 52 mayprovide a 0.25 volt offset from the actual column voltage in oneembodiment.

A pass transistor 56 is controlled by the sample and hold (SH) strobesignal. The transistors 56 conduct when the sample and hold (SH) signalturns on the transistors. The SH strobe signal determines when to latchthe reference voltage level, by turning off the transistors 56. Thesignal from the pass transistor 56 may then be stored for subsequentcomparison for a period of time in a capacitor 58. In one embodiment,the capacitor 58 may be 0.001 microfarads, storing the reference voltagesignal for about 200 nanoseconds for example or whatever time isnecessary to allow the column to change for further comparison to thisstored level. This same function can also be done by a moresophisticated sample and hold.

In one embodiment, the phase change material used in the sensing device12 b may be suitable for non-volatile memory data storage. The phasechange material may be a material having electrical properties (e.g.,resistance) that may be changed through the application of energy suchas, for example, heat, light, voltage potential, or electrical current.

Examples of phase change materials may include a chalcogenide material.A chalcogenide material may be a material that includes at least oneelement from column VI of the periodic table or may be a material thatincludes one or more of the chalcogen elements, e.g., any of theelements of tellurium, sulfur, or selenium. Chalcogenide materials maybe non-volatile memory materials that may be used to store informationthat is retained even after electrical power is removed.

In one embodiment, the phase change material may be chalcogenide elementcomposition from the class of tellurium-germanium-antimony(Te_(x)Ge_(y)Sb_(z)) material or a GeSbTe alloy, such as type 2,2,5although the scope of the present invention is not limited to just thesematerials.

In one embodiment, if the memory material is a non-volatile, phasechange material, the memory material may be programmed into one of atleast two memory states by applying an electrical signal to the memorymaterial. An electrical signal may alter the phase of the memorymaterial between a substantially crystalline state and a substantiallyamorphous state, wherein the electrical resistance of the memorymaterial in the substantially amorphous state is greater than theresistance of the memory material in the substantially crystallinestate.

Programming of the memory material to alter the state or phase of thematerial may be accomplished by selecting the cell through applying arelatively low voltage, such as zero volts to the line 16 and a currentinto the selected column 14, from current source 22 (to reset to ahigher resistance) or 24 (a lower current or slower trailing edge toreset to a lower resistance) thereby generating a voltage potentialacross the memory material. An electrical current may flow through aportion of the memory material in response to the applied voltagepotentials, and may result in heating of the memory material.

This controlled heating and subsequent controlled cooling may alter thememory state or phase of the memory material. Altering the phase orstate of the memory material may alter an electrical characteristic ofthe memory material. For example, resistance of the material may bealtered by altering the phase of the memory material. Either all or aportion of the phase change memory material may be altered during thewriting pulse (i.e. only a portion/region of sensing device 12 badjacent to either the top or bottom electrode may be phase changedduring the write operation). In one embodiment, primarily the portion ofmemory material that undergoes phase change is the region that isadjacent to the smaller electrode contacting the memory element 12 b.The memory material may also be referred to as a programmable resistivematerial or simply a programmable resistance material or memory.

In one embodiment, a voltage pulse with a potential difference of about1.5 volts may be applied across a portion of the memory material byapplying about 0 volts to a line 16 and a current of about 2 ma fromwrite current source 22 into the selected line 14. For example, thevoltage on the selected line 14 is positive relative to the selectedline 16, but the cell or voltages may also be reversed. A currentflowing through the memory material in response to the applied voltagepotentials may result in heating of the memory material. This heatingand subsequent controlled cooling, determined by the write current pulsetrailing edge rate, may alter the memory state or phase of the materialafter it is cooled, from higher to lower resistance or lower to higherresistance, or simply to rewrite the existing state to reinforce it.

In a “reset” state, the memory material may be in an amorphous orsemi-amorphous state and in a “set” state, the memory material may be ina crystalline or semi-crystalline state. The resistance of the memorymaterial in the amorphous or semi-amorphous state may be greater thanthe resistance of the material in the crystalline or semi-crystallinestate. The association of reset and set with amorphous and crystallinestates, respectively, is a convention. Other conventions may be adopted.

Due to electrical current, the memory material may be heated to arelatively higher temperature and then subsequently cooled at a fastrate (for example, less than 10 nsec. edge decay of voltages on thecolumn in terminating the write current) to amorphisize memory materialand “reset” memory material. Heating the volume or memory material to arelatively lower crystallization temperature or reducing the temperatureslowly (e.g. edge rage greater than 100 nsec.) may crystallize usingreduced current and “set” the memory material to a lower resistancestate. Various resistances of memory material may be achieved to storeinformation by varying the amount of current flow and duration throughthe volume of memory material, or by tailoring the edge rate of thetrailing edge of the programming current or voltage pulse (that maycontrol the cooling rate of the selected memory element). Thesetechniques may use feedback by reading the cell and then rewriting it toadjust, thus tailoring the memory cell more precisely to a desiredresistance, and thus allowing storing and sensing more than two levelsin the cells for more efficient data or analog storage.

The information stored in memory material may be read by measuring theresistance of the memory material. As an example, a read current may beprovided to the memory material using the selected row and column and aresulting read voltage across the memory material may be comparedagainst a reference voltage using, for example, the sense amplifier 42.The resulting read voltage on the column may be proportional to theresistance exhibited by the selected memory storage device 12 b when aread current is forced into the column.

In a low voltage or low field regime “L” in FIG. 2A, the device 12 a isoff and may exhibit very high resistance in some embodiments. The offresistance can, for example, range from preferably greater than 50,000ohms to be greater than 10 gigaohms at a bias of about half thethreshold voltage. The device 12 a may remain in its off state until avoltage across it exceeds a threshold voltage (V_(TH)) or until acurrent exceeding a threshold current (I_(TH)) switches the device 12 ato a highly conductive, low or dynamic resistance “on” state calleddV/dI region that, when extrapolated, will intercept the X axis at theholding voltage V_(H)(ots), as indicated in FIG. 2A. After turn-on, thevoltage across the device 12 a drops to a lower voltage, called theholding voltage V_(H), and remains very close to this holding voltagealmost regardless of the column current since the dynamic on resistanceis relatively low, frequently less than 1000 ohms (in series with thethreshold device 12 a holding voltage). In one embodiment of the presentinvention, as an example, the threshold voltage may be on the order of3.1 volts and the holding voltage may be on the order of 2.9 volts. Thisrelatively high holding voltage that is near the threshold voltage maybe achieved by using three OTS devices in series as select device 12 a,each with a threshold voltage of 1V, about equal to its holding voltage.Alternatively, a higher V_(TH), lower leakage select device 12 a may beused in series with a lower V_(TH) select device 12 a, having higherleakage. The combined select device may have a reduced differencebetween V_(TH) and V_(H) to better assure that the snapback does notforce excessive current (Isafe) though the memory element 12 b that willincrease its resistance during reading. In one embodiment, the Vthotsmay be increased to be greater than Vhots by more than Vthoum. Then,when the ots thresholds, the snapback voltage difference from Vth to Vhwill exceed Vthoum. Hence, when the ots selects a reset bit, the resetbit will also threshold when the ots thresholds. By allowing Vsnap toexceed Vthoum, a single device with lower leakage may be used. Forexample, Vthots may be 3V and Vhots 1V, even if the Vthoum is 1V orless.

After passing through the snapback region, in the on state, the device12 a voltage drop remains close to the holding voltage as the currentpassing through the device is increased, even if at a relatively high,read or write current level. Above a relatively high current level(density) the device remains on but displays a finite and increasingdynamic resistance, with the voltage drop across 12 a increasing withincreasing current due to the IR drop across the dynamic resistance.

The device 12 a may remain on until the current through the device 12 ais reduced below a characteristic holding current value that isdependent on the type and area of the material, and may be impacted bythe top and bottom electrodes utilized to form the device 12 a, as wellas the magnitude of capacitance on the column line.

In some embodiments of the present invention, the threshold device 12 adoes not change phase. It remains permanently amorphous and itscurrent-voltage characteristics may remain the same throughout itsoperating life.

As an example, for a 0.5 micrometer diameter device 12 a, with athickness of approximately 750 Angstroms, formed of TeAsGeSSe havingrespective atomic percents of 16/13/15/1/55, the holding current may beon the order of 0.1 to 1 micro-amps (uA) in one embodiment. Below thisholding current, the device 12 a turns off and returns to the highresistance regime at low voltage, low field L in FIG. 2A. The thresholdcurrent for the device 12 a may generally be of the same order as theholding current, or preferably greater to avoid oscillation if driven bya high impedance current source. The holding current may be altered bychanging process variables, such as the top and bottom electrodematerial and the chalcogenide material. The device 12 a may provide high“on current” for a given area of device compared to conventional accessdevices such as metal oxide semiconductor field effect transistors orbipolar junction transistors or semiconductor diodes. However, suchother select devices may also be used in some embodiments.

As shown in FIG. 2B, the set bit is relatively low resistance beginningat the origin and sloping up to intercept the snapback of the reset bit,where the dynamic resistance tends to be lower for increasing current.Reducing the current will retrace the origin if the bit is set. If thebit is reset, as the current is reduced the voltage is reduced untilless than I_(H)(oum), where the voltage then increases along the lowercurve towards V_(TH)(oum).

Referring to FIG. 4A, the column voltage level over time is shown for aselected column 14 c and row in accordance with one hypotheticalembodiment. In standby or when deselected, the column and row voltagesmay approximately equal to V/2, where V is related to the thresholdvoltages of the devices 12 a and 12 b and may be adjusted die to die.For improved voltage margin at the expense of leakage, the columndeselect may be V/3 and the row deselect 2V/3. For example, V may be 4Vif the threshold voltage of the device 12 a is typically 3V with V_(H)of 1V, and if the threshold voltage of the device 12 b is typically 1Vwith a V_(H) of 0.5V.

While the selected column voltage goes high, the selected row voltagegoes low (unless the select device is an N-channel transistor, where therow line goes high for select). The selected row 16 has an initiallyhigh voltage which falls to a steady low voltage as indicated. Thedeselected column 14 has a relatively low voltage which increases as thecolumn is selected. The column selection is indicated in FIG. 4B whereit is shown how the selected column current steps up, causing theselected column voltage to rise after the column is selected. The readcolumn current is above the threshold currents of the threshold device12 a and memory element 12 b.

The selected column line 14 c is first charged from a deselected voltageto a voltage forced by a read current through the bit that starts attime t₁ in FIG. 4B. The read current (e.g., 70 microAmps) is greaterthan the threshold current of the memory element 12 b so selected columncharging is achieved relatively quickly with this relatively higher readcurrent. Better speed may be achieved in some embodiments with evenhigher currents since the column voltage charging rate is proportionalto the current (dV/dT=I/C).

The column voltage first peaks at B when the select device 12 a triggersfrom V_(TH) to V_(H). If the bit is set, the voltage may drop back to alower voltage due to the relatively lower resistance of a set bit. Ifthe bit is reset, the voltage may initially decrease, but will continueto increase thereafter from B to D, when the reset memory element 12 btriggers to a lower holding voltage. If the difference between V_(TH)and V_(H) of the selected device 12 a is greater than V_(TH) of thememory element 12 b then both the select device 12 a and reset memoryelement 129 will trigger at B.

The resulting voltage VR is sampled at time t₂ in the dV/dI region ofboth the select device 12 a and memory element 12 b (indicated in FIGS.2A and 2B) and after the column is near its final voltage,Vfinal(V_(H)(ots)+V_(H)(oum)×Iread×((Rdyn(ots)+Rdyn(oum)), so it will bea similar voltage almost regardless of the memory element 12 b bit state(high or low resistance stored in the memory cell 12). The dV/dI regionis a low impedance region reached after passing through the snapbackregion of both elements 12 a and 12 b. By forcing a read current greaterthan the I_(TH) of each device 12 a and 12 b, both are forced into theirrespective dV/dI regions.

After the column approaches Vfinal, this resulting column voltage issampled at t2, reduced by an offset voltage (such as 0.25V), and thenthis voltage is held as a reference voltage by the reference voltagegenerator 40 for input to the comparator 42. After it is latched at 52,this voltage VREF is held until after t4 for comparison to the columnvoltage as indicated in FIG. 4C.

After the column voltage is sampled and held, the read current into thecolumn is reduced at time t₃, after t₂. This starts the period of timefor comparing the declining column voltage with the reference voltage(VREF) after the time t₃ when the column current is reduced. Once thecurrent is reduced, the resulting column voltage after a period of time(t₄−t₃) is compared to the reference voltage by the comparator 42 as arelatively more positive or negative voltage, with the result latched bystrobing the data at time t₄ as shown in FIG. 4D.

The reference voltage (VR) may be taken from the column, in anotherembodiment, where the row voltage is not pulled all the way to ground(to minimize the effect of row leakage to deselect columns and grounddrop), or relative to change in voltage voltage where the row voltagegoes high when selected (because the cell select device 12 a is anN-channel transistor, for example). By sampling the voltage at t2, theeffect of variations in ground voltage, row voltage, and holdingvoltages of the ots and oum are minimized Other variations are alsopossible.

With a read current that is greater than the threshold current (I_(TH)in FIG. 2B) of element 12 b, usually the device 12 a may trigger (seepoint B in FIG. 4A) before the element 12 b (see point C in FIG. 4A)because the selected device's threshold current may be less than thememory element's threshold current in some embodiments. If the bit isset (low resistance), the RC time constant on the column after thedevice 12 a triggers is low because the set resistance is low. If theelement 12 b is in the high resistance state, the column continues tocharge fast since the final voltage is much greater than the sum ofholding voltage of the device 12 a and the threshold voltage of thedevice 12 b, until device 12 b triggers. The dynamic resistance of theoum is still high until the memory element 12 b triggers so the finalvoltage towards which the column charges until the memory elementtriggers is still very high after the select device 12 a triggers.However, if Vsnap(ots) is greater than Vh(oum), then the device 12 bwill threshold and the voltage reached by both the set and reset bitwill be about the same (if Iread×Rset is about equal to Vhoum). Then, inFIG. 4A, the Reset waveform will be about the same as a set waveformuntil t3.

Once the bit is accessed after the device 12 a triggers, a set bit is ina low resistance state and the column reaches its final voltage (Vfinal)quickly due to its low RC time constant. For a higher resistance resetbit, the column RC is high, but the column continues to rise rapidlysince the final voltage (Vfinal) charging voltage is still very highcompared to the column voltage because Iread×Rbit is high until thereset bit triggers into a low resistance state. As the voltage acrossthe element 12 b exceeds its threshold (if it is reset), the bittriggers into a low dV/dI state, and decays to about the same voltage asthe voltage from a set bit, with a read current greater than the device12 b oum threshold current. Thereafter both the oum and ots beingtriggered (if not at about the same time), the actual Vfinal is reachedquickly since both are in a low dynamic resistance state, so RC is low.

After both elements 12 a and 12 b have triggered, the bit voltage(whether set or reset) is approximately V_(HA)+V_(HB)+Iread×(dV/dI(device a)+dV/dI (device b)), where V_(HA) and V_(HB) are the holdingvoltages of the device 12 a and element 12 b, respectively. Thispresumes that Iread×Rset for a set bit is about the same as the Vhoumthat a reset bit thresholds to. This voltage D, set by dV/dI×readcurrent plus holding voltage of the ots, and plus holding voltage of theoum if it is reset, is achieved rapidly once both trigger since thedynamic resistance is low once both the select device 12 a and memoryelement 12 b trigger into the dV/dI region.

While the resistance is low in the dV/dI portion of the IV curve forreset memory 12 b after it thresholds, the amorphous bit state isretained as shown by the memory element 12 b returning to the higherresistance state after the current through memory element 12 b fallsbelow I_(H) (so long as the current through the bit does not exceedImelt at which current the bit starts to change from Ireset to Iset).Once triggered, the reset bit stays in a relatively low resistance likea set bit until the current is returned to less than the thresholdcurrent. This final voltage D achieved after the column line voltagestabilizes in the dV/dI portion of the IV curve of the cell elements iscalled the “precharge voltage” used to generate the reference voltage.

After the precharge voltage D is established and stable on the columnline, the column line voltage VR can be sampled to establish a lowerreference voltage (VREF). The selected column bit voltages VR and VREFare inputs to the comparator 42.

The column current is reduced or turned off at the time t₃ in FIG. 4B,and the column rapidly discharges toward the memory element 12 b holdingvoltage, since dV/dI is a relatively low resistance for both set andreset bits until, for that logic case, the reset bit unthresholds. Readcurrent may be adjusted so that Iread×Rset is about equal to Vhoum fordevice 12 b. Below V_(H) for the set bit case, the resulting voltageacross a set bit continues to decay rapidly (region E in FIG. 4A). Thusfor a set bit, after decreasing the read current, the column voltagedecays rapidly from its starting precharge voltage to nearly zero acrossthe memory element 12 b with the column at a voltage equal to Vrowdriver+select element 12 a V_(H). The voltage stays at Vhots if Ireadremains greater than Ithots, or may decay further below Vhots for lesscurrent, but very slowly since the ots has high resistance as itunthresholds.

In contrast for a reset bit, the column line stops discharging rapidlyonce the device 12 a holding voltage and current is reached and ituntriggers into a high resistance state. Further voltage decaythereafter is at a very slow rate, such as microseconds (region F inFIG. 4A).

Thus for both states, the initial voltage decay rate (dV/dt) (region E)is set by the resistance of the dV/dI portion of the IV curve untilvoltage across memory element 12 b is less than V_(HB). Then the decayrate (region F) changes to a rate set by the resistance of the set bitbelow the holding voltage, often somewhat slower even for a set bitbecause the set resistance is moderately greater than dV/dI and muchslower for a reset bit once it untriggers as the current through thememory falls below I_(H).

With VREF set between a voltage of V_(HA) and a higher voltage ofV_(Hots)+V_(Houm), the data may be strobed, as indicated in FIG. 4D,after waiting some portion of the set bit RC time constant, where R isthe set bit resistance and C is column capacitance. Preferably, thistime should be short enough so the reset bit case has not significantlydischarged the column line below its Vh voltage, and the set bit casehas discharged the column close to Vhots so that 90% of the voltagedifference from the voltage at T2−Vhots has decayed.

In some embodiments, using this approach may reduce the effect ofvariation in device 12 a holding voltage, bit to bit, by first samplingthe column voltage before the column current is turned off for finalcomparison to the resulting sampled voltage after the column current isturned off or substantially reduced to a current less than I_(Houm), butperhaps greater than I_(Hots).

The reference for comparison to the resulting bit voltages is firstreferenced using the voltage of the bit being read, which is thensubtracted when the later comparison is made, reducing cell to cellvariation. Accordingly, voltage margins may be improved in someembodiments. Further, the precharge voltage may be established rapidlysince a current greater than the threshold current is used to create thesampled voltage and its variation may be reduced or minimized since thedynamic resistance in this region of the IV curve for both the device 12a and element 12 b may be a minimum.

Further reduction in read delay may be accomplished by allowing theVthots to be greater than Vhots by more than Vthoum. Then, the delayfrom B to C is eliminated, so the delay from t1 to t3 is reduced.

To further improve margin, read current mirror may use an extra columnresistance R in series, that may be placed to minimize area or placed oneach column for even more effectiveness below the high capacitance node.The result is that the voltage VR on the column where the dischargeslows for a reset bit is at a higher voltage relative to that achievedrapidly by a set bit, so that the read window is amplified in someembodiments.

The voltage across the optional resistor R may provide added read marginwhich can be adjusted with the size of the resistor to fit the thresholdcurrent for a given chip, using the non-volatile portion of the memoryor the fuse redundancy repair circuit to adjust, along with the VREF, toimprove margins. The magnitude of the resistance, as is apparent to onereasonably skilled in the art, is limited to that allowed by performancerequirements to the delay increase in the RC discharge time for a setbit. More voltage margin is provided at increased R but at the expenseof increased read access delay.

The memory element 12 b may reasonably retain its memory cell state fora useful number of cycles even though the current drives the reset bitinto the low resistance dV/dI state. However, repeated reads thatthreshold (trigger) the reset bit) will gradually, until the bit isreset, decrease the memory element 12 b threshold voltage and storedresistance for currents below I_(TH). Once the resistance degrades toomuch for adequate read voltage margin, the reset bit must be re-written(refreshed) to avoid a read disturb. Refresh is accomplished by firstreading a bit, and those in the reset (high resistance state can then berewritten at the end of the read cycle using the faster reset cycle(extending the read cycle time accordingly).

Limiting the current and use of low capacitance loads may assist dataretention and reduce bit rewriting frequency, extending endurance andreducing overhead time spent rewriting during a read. Also, cycling thebit, including with a few read cycle may remove the threshold relatedsnapback. If this is done fifteen cycles during the write cycle and thebit verified, better data retention can be achieved. Further, the bitscan be read on power down or power up to restore those degraded by readcycles. Alternatively, the entire block of memory can be refreshedperiodically, such as every second, depending on the number ofworst-case read cycles of a single bit. Alternately, since the write ofa reset bit is relatively fast, each time a bit is read and found to bereset, the bit may be rewritten during the read cycle—retaining the goodread access delay but possibly increasing read cycle time. Repeatedlyrewriting a reset bit will adversely affect endurance so normal blockreallocation techniques used for flash may be applied to repeated readsor writes of a given bit.

Using the techniques described herein, margins may be improved byavoiding the reduction in read window margin due to variation in device12 a hold voltage. This may improve yield and field reliability in someembodiments.

A voltage reflective of cell select and memory hold voltages is firstgenerated for sample/hold with the column read current to create areference voltage, and then the read current is turned off or reducedfor comparison to the sampled reference voltage to improve margins bysubtracting the hold voltage which may vary bit to bit, so the resultingcomparison relates more to the memory resistance more reflective of cellstate. In some embodiments, by using the dV/dI region D in FIG. 4A toset the reference voltage, holding voltage variation effects on readmargins may be reduced and read performance improved by only using thememory in a low impedance mode.

Referring to FIG. 5A, a read cycle is shown for both set and reset bits.As a column and row are selected, the column voltage increases over timeuntil the device 12 a thresholds (as indicated) and the device 12 bthresholds (as indicated). Then the voltage drops and remains steady,such as in the period before Trap Vref in FIG. 5C (before Latch goeshigh). After the reference voltage is trapped (FIG. 5C) and the columncurrent is reduced (FIG. 5B) to less than Ihoum, a reset bit voltageincreases towards Vthoum+Vhots, while a set bit decreases relative to areference voltage V_(REF) towards Vhots. The sensed voltages can becompared to the reference voltage to determine whether the bit is eitherset or reset.

Referring to FIG. 5B, a column current is forced on the column during aread cycle. That column current ramps up initially to a first level andthen ramps down to an intermediate level. The first level is such thatthe read current is greater than the threshold current of the device 12b and that current is greater than the threshold current of the device12 a. The second read current level is less than the holding current ofthe device 12 b, but greater than the holding current of the device 12a. Preferably, I read should also be less than the threshold current ofoum device 12 b. To further increase the read voltage, the current maybe first lowered to be less than Ihoum and then increased, whileassuring still less than Ithoum.

Referring to FIG. 5C, the latching of the reference voltage VREF isillustrated. In response to a latch pulse, the reference voltage may betrapped for comparison to the sensed bit voltage.

Next, referring to FIG. 5D, a pulse is utilized to latch out the senseddata. In other words, in response to the trapped data pulse asindicated, the determined bit state may be latched out.

Referring to FIG. 5E, a pulse may be provided to make a marginmeasurement if needed. Finally, in FIG. 6F, if the measured margin isinsufficient, another write current pulse may be provided. In such case,the process may be repeated to read the bit again, hopefully with bettermargin the next time.

Regardless of the read current, shown in FIG. 5B, the snapback voltage(FIG. 2A) of the device 12 a may be greater than the threshold voltage(FIG. 2B) of the device 12 b. When reading the bit, the bit will bethresholded if it is reset. Then, the read voltage (FIG. 5A) across thecell 12 is the holding voltage of the device 12 a (FIG. 2A) plus theholding voltage of the device 12 b (FIG. 2B) plus R_(DYN)I_(READ) whereR_(DYN) is dV/dI(OUM) (FIG. 2B)+dV/dI(OTS) (FIG. 2A).

However, if the bit is set, the voltage across the cell 12 is theholding voltage of the device 12 a plus the set resistance times theread current, where the read current is adjusted to be sure this voltageis at least 200 millivolts less than the reset bit voltage. Then, thebit can be refreshed by writing to the state of the bit that is read,either each cycle or after checking margin to see if the write cycle isneeded. That is, the resistance of a set bit can be measured to insurethat it is less than a predetermined maximum resistance for a set bit.The reset bit resistance may be measured after reading to insure that itis more than a minimum predetermined reset resistance.

Alternatively, to improve margin, the reading of the bit can be doneafter the transient current on the device 12 b falls below the holdingcurrent of that device, so that a reset state device 12 b isuntriggered. The device 12 a would still be triggered because theapplied forced read current exceeds its holding voltage. Then, theresistance of the selected bit may be measured to be sure that it isstill greater than the minimum reset resistance. Signal of a reset bitcan be increased by increasing the current after it unthresholds, suchas to less than Ith(oum).

Allowing the reset bit to untrigger, may result in more voltagedifference between the set and reset bits. As the read cycle proceeds,the voltage across device 12 a exceeds its threshold voltage so that thedevice 12 a snaps back to its holding voltage (FIG. 2A). This snapbackforces more than its threshold voltage across the device 12 b, if it isreset, so the voltage across device 12 b triggers to its holding voltageplus the column current times dV/dI. Then, the current decays as thetransients die out and the current falls to the read current (FIG. 5B).If the read current is less than the holding voltage of the device 12 b,then the voltage falls along dV/dI (FIG. 2B) until the device 12 buntriggers. Then, the voltage across the device 12 b may increasetowards its threshold voltage, but does not exceed its threshold voltageif the forced current is less than the threshold current of the device12 b.

In some embodiments, a single threshold device 12 a may be used inseries with a memory element 12 b to simplify the manufacturing processand allow the use of a lower leakage, thicker select device 12 a in thecell 12. The memory cell 12 may include a select device 12 a in serieswith a memory element 12 b, where the select device 12 a is allowed tobe thicker and have a greater snapback voltage than the thresholdvoltage of the memory element 12 b. As a result in some embodiments, thestack of the devices 12 a and 12 b may be thinner since a single selectdevice is utilized. By using a single thicker device with more snapback,leakage may also be reduced to deslected rows since thicker ots deviceswith more snapback tend to less leakage.

On each read cycle, the memory element 12 b is read. If the bit isthresholded, it can be read while thresholded by being sure the voltageacross a reset bit is adequately more than the holding voltage of thememory element 12 b. Or, the memory element 12 b may be triggered whenthe select device 12 a snaps back. Then, the read current can be loweredto be above the holding current of the select device 12 a but less thanthe holding current of the memory element 12 b.

When the current in the memory element 12 b falls below the holdingcurrent of that device, the reset element 12 b untriggers.

After triggering device 12 b by forcing current greater than Ith, thenforcing a read current less than the holding current of the memoryelement 12 b and then increasing the current to be less than Ithoum, butgreater than the holding current of the memory element 12 a, can forcemore voltage than the memory element's holding voltage and less than thememory element's threshold voltage (after transient current caused bytriggering the device 12 a decays to become the read current), allowingmore read voltage and margin compared to the read voltage generatedacross a triggered reset bit.

After reading, the resistance of the selected bit may be checked and thebit rewritten or refreshed if necessary (FIGS. 5E and 5F). This may alsobe done on each cycle or periodically, such as at power-up.

Turning to FIG. 6, a portion of a system 500 in accordance with anembodiment of the present invention is described. System 500 may be usedin wireless devices such as, for example, a personal digital assistant(PDA), a laptop or portable computer with wireless capability, a webtablet, a wireless telephone, a pager, an instant messaging device, adigital music player, a digital camera, or other devices that may beadapted to transmit and/or receive information wirelessly. System 500may be used in any of the following systems: a wireless local areanetwork (WLAN) system, a wireless personal area network (WPAN) system,or a cellular network, although the scope of the present invention isnot limited in this respect.

System 500 may include a controller 510, an input/output (I/O) device520 (e.g. a keypad, display), a memory 530, a wireless interface 540,and a static random access memory (SRAM) 560 and coupled to each othervia a bus 550. A battery 580 may supply power to the system 500 in oneembodiment. It should be noted that the scope of the present inventionis not limited to embodiments having any or all of these components.

Controller 510 may comprise, for example, one or more microprocessors,digital signal processors, micro-controllers, or the like. Memory 530may be used to store messages transmitted to or by system 500. Memory530 may also optionally be used to store instructions that are executedby controller 510 during the operation of system 500, and may be used tostore user data. The instructions may be stored as digital informationand the user data, as disclosed herein, may be stored in one section ofthe memory as digital data and in another section as analog memory. Asanother example, a given section at one time may be labeled as such andstore digital information, and then later may be relabeled andreconfigured to store analog information. Memory 530 may be provided byone or more different types of memory. For example, memory 530 maycomprise a volatile memory (any type of random access memory), anon-volatile memory such as a flash memory, and/or memory 10 illustratedin FIG. 1.

The I/O device 520 may be used to generate a message. The system 500 mayuse the wireless interface 540 to transmit and receive messages to andfrom a wireless communication network with a radio frequency (RF)signal. Examples of the wireless interface 540 may include an antenna,or a wireless transceiver, such as a dipole antenna, although the scopeof the present invention is not limited in this respect. Also, the I/Odevice 520 may deliver a voltage reflecting what is stored as either adigital output (if digital information was stored), or it may be analoginformation (if analog information was stored).

While an example in a wireless application is provided above,embodiments of the present invention may also be used in non-wirelessapplications as well.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A method comprising: reading a cell on a line by triggering the phasechange memory cell; providing a reference level from said line;comparing said reference level to a level on said line after a timedelay; using a reference level derived from the voltage on the line; andcomparing the reference voltage generated before a read current to theselected line is reduced, to a reference voltage generated on theselected line after the read current is reduced and then increased. 2.The method of claim 1 including obtaining said reference level when saidcell is in a dV/dI region.
 3. The method of claim 2 including storingsaid reference level from the line for comparison to a level on saidline at a later time.
 4. The method of claim 3 including comparing saidlevels after turning off the current to said address line.
 5. The methodof claim 1 wherein reading a cell on a line includes driving a linecurrent greater than the threshold current of said cell.
 6. The methodof claim 5 including using a cell with a phase change memory element anda threshold device and driving a column current greater than thethreshold current of said element.
 7. The method of claim 6 includingusing a chalcogenide threshold device that is not programmable.
 8. Themethod of claim 1 including refreshing said cell after reading saidcell.